#1-72 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
#1-72 UNF tap drill — 75% thread
#53
0.0595" · 1.5 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.0730 − (0.01299 × 75) / 72 = 0.0595"
The nearest standard drill above that is #53 at 0.0595".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.0640" | #52 (0.0635") | 1.6 mm |
| 65% | 0.0613" | 1/16 (0.0625") | 1.6 mm |
| 75% | 0.0595" | #53 (0.0595") | 1.5 mm |
| 85% | 0.0577" | #53 (0.0595") | 1.5 mm |
Which percentage should you actually use?
75% for through-holes in mild steel and aluminium. 65% for blind holes, stainless, titanium and anything where the tap is at risk. 50% for hand-tapping deep holes. The strength you give up going from 75% to 60% is around 5%; the tapping torque you give up is around 30%. In practice the threads almost never fail — the tap does.
75% for through-holes in mild steel and aluminium. 65% for blind holes, stainless, titanium and anything where the tap is at risk. 50% for hand-tapping deep holes. The strength you give up going from 75% to 60% is around 5%; the tapping torque you give up is around 30%. In practice the threads almost never fail — the tap does.